Our research focuses on the identification and characterization of genes critical for structure and function of sensory hair cells in the cochlea and vestibule. Spontaneous mutations that cause balance dysfunction and hearing loss are ideal tools to identify genes important for the functioning of the inner ear and to elucidate their role in these sensory systems. Towards this end we concentrate our efforts on three deafness mutations: jackson circler (jc), jerker (je) and Varitint-waddler (Va). By auditory-evoked brain stem response analyses we showed that in these strains hearing impairment is completely penetrant and fully expressed in three to four week old animals. The vestibular phenotype is also fully expressed in je and Va mutants, but seems to vary in jc. To search for hearing and balance modifiers we outcrossed each of the mutations onto different genetic backgrounds. To identify their molecular identity we analyzed large segregating intersubspecific intercrosses. We constructed physical BAC contigs for each of the mutated loci and we evaluated candidate genes. The molecular cloning of these mutations should give us new insights in the development and function of the mammalian cochlea. High-resulution genetic and physical mapping of varitint-waddler identified two new members of the mucolipin gene familiy Mcoln2 and Mcoln3 in the critical interval. We found missense mutations in Mcoln3 which are responsible for deafness and pigmentaton defects in Va. Mcoln3 shows sequence and motif similarities to the transient-receptor-potential (TRP) family of ion channels. Mcoln3 localizes to cytoplasmic compartments and stereocilia of outer and inner hair cells in the organ of Corti. Based upon its motif structure and expression domain, we suggest that Mcoln3 is involved in the regulation of ion homeostasis in hair cells and melanocyte. These studies identified a new molecular link between deafness and pigmentation defects. Finally, Mcoln2 and Mcoln3 are candidate genes for hereditary and/or sporadic forms of neurosensory disorders. Work is in progress to characterize the cellular and molecular function of Mcoln3 in cochlea hair cells. [unreadable] We recently reported on the identification of a new allele of the protocadherin 15 gene in the mouse. Mutations in this gene cause deafness and balancing defects. This mouse model might be valuable in studying cochlea and retinal pathalogy in Usher syndrome type 1F.[unreadable] In order to understand the genetics and mechanistics of polygenic, non-mendelian inheritance of hearing loss, we surveyed outbred strains for hearing function. Outbred strains represent a genetic spectrum different from the common inbred strains and show a greater degree of allelic heterogeneity. They are models to isolate genetic interactions and to identify new deafness alleles. We found that the BlackSwiss outbred strains undergoes early-onset slow progressing hearing loss. The segregation pattern in intercross and backcross was consistent with a polygenic inheritance. Genome-wide linkage analyses on backcross and intercross populations identified and localized three quantitative trait loce (QTLs) underlying hearing loss in Black Swiss mice. A major QTL localized to chromosome 10 (named ahl5) and two small-effect QTLs localized to chromosome 13 (ahl6) and chromsome 18 (ahl7). Furthermore, four outbred strains with distinct hearing loss profiles were characterized.[unreadable] In a recent study, we refined the phenotypic and genetic parameters of the original jc mutation and characterized a new mutant allele, jc2J. In open-field behavior tests, homozygous jc mutants exhibited abnormal circling and ambulatory behavior that was indistinguishable from that of phenotypically similar mutants with defects in the vestibule of the inner ear. The jc/jc and jc2J/jc2J mice had stable elevated auditory-evoked brainstem response (ABR) thresholds at the 16 kHz stimulus of 88 ? 9 dB sound pressure leveels (SPL) and 43 ? 11 dB SPL, respectively. Peak latencies and peak time intervals were normal in jc mutants. The jc mice showed no measurable distortion-product-otoacoustic-emissions (DPOAEs) above the system noise floor. In the mutant cochlea, the apical turn failed to form due to the developmental growth arrest of the cochlear duct at the level of the first turn at gestational day 13.5. In a large intrasubspecific intercross, jc localized to a 0.2 cM interval at position 25cM on chromosome 10, which is homologous to the human 6q21 region. On CZECHII/Ei and CAST/Ei backgrounds jc/jc mutant hearing thresholds at the 16 kHz stimulus were significantly lower than those observed on the C57BL/6J background, with means of 62?22 dB SPL and 55?18 dB SPL, respectively. Genome-wide linkage scans of backcross, intercross, and congenic progeny revealed a complex pattern of genetic and stochastic effects.